1. Field of the Invention
The invention is directed to control of prefire timing, ink ejection power, and pulse width modulation for a print head used with an ink jet printing apparatus. More particularly, the invention relates to such control based on ink ejection and prefire timing and changes in a number of driven print head nozzles.
2. Description of the Related Art
Ink jet printers print images by ejecting ink through nozzles of print heads onto recording media. In order to form different images, different nozzles are driven so as to eject ink while the print head or print heads are scanned across a recording medium. Depending on the image that is recorded, some nozzles may be unused for multiple scans. Residual ink in the unused nozzles tends to dry or coagulate, clogging the nozzles. Then, when then ink jet printer attempts to use the clogged nozzles, the nozzles might not eject ink properly, resulting in poor image quality.
One conventional technique for maintaining unclogged nozzles is to periodically prefire the nozzles. Prefiring the nozzles involves moving the print head or print heads to a home position in the printer and then ejecting ink from the nozzles, thereby clearing the nozzles of drying or coagulating ink. Convention systems perform this prefire operation at a regular interval.
Moving the print head or print heads to the home position and ejecting ink from the nozzles takes considerable time. As a result, performing the prefire operation at a short time interval unacceptably slows the image forming operation. However, performing the prefire operation at a long interval may not be sufficient to maintain unclogged nozzles and high image quality. In particular, unexpected interactions between high-end host processors and printing devices that use long intervals have been foung to result in significant image degradation, as explained below in section 9.0 with reference to FIGS. 9-2a through 9-2d. Accordingly, there exists a need for control of prefiring and nozzle operation that provides acceptable image formation speed while maintaining unclogged print head nozzles and high image quality.